1. Field of the Invention
The present invention relates to a cesium-lithium borate crystal. More particularly, the present invention relates to a cesium-lithium borate crystal which is useful as wavelength converting nonlinear optical crystal or the like for a laser oscillator or an optical parametric oscillator used in ultraviolet-ray lithography, laser fine processing and laser fusion and permits improvement of uniformity of refractive index, improvement of mecahnical, chemical and optical properties, and prevention of occurrence of cracks.
2. Description of the Related Art
In a conventional laser oscillator used in ultraviolet-ray lithography, laser fine processing or laser fusion, it is necessary to obtain stable ultraviolet rays efficiently, and wherein the a method of efficiently obtaining ultraviolet rays through wavelength conversion of a light source using a nonlinear optical crystal is attracting the general attention as a method for this purpose.
In a pulse YAG laser oscillator as a laser oscillator, the wavelength of a light source is converted with the use of a nonlinear optical crystal to generate triploid harmonic (wavelength: 355 nm) or tetraploid harmonic (wavelength: 266 nm).
Many contrivances have been made regarding a wavelength converting nonlinear optical crystal indispensable for generating ultraviolet rays. For example, crystals of such borates as betabarium metaborate (.beta.-BaB.sub.2 O.sub.4), lithium triborate (LiB.sub.3 O.sub.5) and cesium triborate (CsB.sub.3 O.sub.5) are known. Such a wavelength converting nonlinear optical crystal for generating ultraviolet rays permits transmission of a light having a wavelength of up to 200 nm, and has a high nonlinear optical constant
However, in .beta.-BaB.sub.2 O.sub.4, one of such wavelength converting nonlinear optical crystals, easy occurrence of phase transition upon melt growth during the process of preparation makes it difficult for crystal growth to take place, and a narrow angular allowance leads to a low generality.
In LiB.sub.3 O.sub.5 or CsB.sub.3 O.sub.5, another of the wavelength converting nonlinear optical crystals, growing period is very long for flux growth during the process of manufacturing, and phase alignment is possible only for a light of a wavelength of up to about 555 nm. It is therefore applicable for the generation of triploid harmonic (wavelength: 355 nm) available by an Nd-YAG laser, for example, but is defective in that it is not applicable for the generation of tetraploid harmonic (wavelength: 266 nm).
Under these circumstances, the present inventors developed a novel cesium-lithium borate crystal, a high-performance wavelength converting nonlinear optical crystal, as a material capable of overcoming the defects in the conventional art, which permitted conversion of a light of a shorter wavelength through transmission, gave a high conversion efficiency, and had wide temperature allowance and angular allowance, and have proposed anew this crystal itself, a method of manufacturing the same and a method of utilizing the same (Japanese Provisional Patent Application JP-A-7-128698). This cesium-lithium borate crystal including composition-substituted crystals was clarified to be an epoch-making one, and is attracting the general attention as an optical material bringing about new developments.
As a result of studies carried out by the present inventors, in spite of its excellent properties, the cesium-lithium borate crystal is now known to have problems in that it tends to be deteriorated because of its hygroscopicity and water solubility, and in that residual processing distortion tends to cause cracks.
The present invention aims created to solve these problems and has an object to permit improvement of mechanical, chemical and optical properties of a cesium-lithium borate crystal, a high-performance wavelength converting nonlinear optical crystal, and composition-substituted crystals thereof, which can transmit a light of a shorter wavelength and convert the wavelength, have a high conversion efficiency and wide temperature allowance and angular allowance.